Induction heating is a rapid and easily controllable heating method for heating an electrically conducting metal or metal alloy workpiece, and can provide sufficient energy to melt the workpiece and maintain it in the molten state. An induction heating apparatus generally includes an inductor, such as an electrically conductive copper coil, that surrounds the workpiece. When the inductor is subjected to a varying electromagnetic field, a varying current is generated within the inductor, which induces an electromotive force in the workpiece. The induced electromotive force results in the generation of an electric current in the workpiece, and the internal resistance to the current in the workpiece heats the workpiece.
An example of a coil-type induction heating apparatus is described in U.S. Pat. No. 5,588,019 to Ruffini et al. entitled "High Performance Induction Melting Coil," which issued on Dec. 24, 1996. The induction-melting coil surrounds a crucible for holding the workpiece. Magnetic flux concentrators that are fabricated from a low reluctance composition are placed around the induction coil. These flux concentrators concentrate the magnetic flux generated by the current carrying induction melting coil at the workpiece. This allows the workpiece to be heated efficiently since less current is required to heat and melt the workpiece than by using just the induction coil. For a workpiece having a complex shape, flux concentrators can direct electromagnetic field energy to areas of the workpiece that are inaccessible to just the induction coil. Flux concentrators also minimize the inductive heating of other components of the apparatus. The induction heating apparatus is also provided with a cooling system to cool the flux concentrators since they are known to lose permeability when heated to high temperatures.
Various methods for making magnetic flux concentrators are known. For example, U.S. Pat. No. 4,776,980 to R. S. Ruffini entitled "Inductor Insert Compositions and Methods," which issued on Oct. 11, 1988, describes compositions used to make inductor inserts, i.e. magnetic flux concentrators. A high purity, disk shaped, annealed, iron powder is treated with phosphoric acid. This treatment provides electrical insulation between the iron particles of the powder, which reduces electrical current, known as "eddy currents" between the iron particles. This results in a reduction in heat generated in the flux concentrators during operation. The treated iron powder is mixed with a polymeric resin binder, and a mold release agent may also be added. The mixture is dried to a powder and pressed in a die to form a body. The body is cured at 150-500.degree. F., and then sanded to produce the magnetic flux concentrator.
Another method for making magnetic flux concentrators is described in U.S. Pat. No. 5,828,940 to T. J. Learman entitled "Formable Composite Magnetic Flux Concentrator and Method of Making the Concentrator," which issued on Oct. 27, 1998. A putty containing electrolytic iron powder, carbonyl iron powder, a binder, and catalysts is prepared. The putty is vibrated under compression to remove air, molded into a body, embedded with hollow elements, and heated to harden the body and produce the magnetic flux concentrator. The hollow elements are a part of a heat removal system to cool the flux concentrator during operation.
Generally, magnetic flux concentrators are provided with a cooling system to remove heat from the concentrators during operation because excessive heat may lead to decomposition of the polymeric binders and to a reduction in the permeability of the magnetic flux concentrator. Clearly, magnetic flux concentrators that can be operated at elevated temperatures without losing substantial permeability are highly desirable.
Therefore, an object of the present invention is a process for making iron-carbon compacts that can be used as, or fabricated into, magnetic flux concentrators.
Another object of the present invention is a process for making magnetic flux concentrators that maintain an operational permeability at temperatures higher than those for conventional flux concentrators containing polymeric resin binders.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.